This invention relates to an apparatus for measuring the heat flux at the surface of a solid material. More particularly, the present invention relates to a particular configuration of heat flux gage made by plating thermocouple materials through holes in a thermal resistance layer.
Heat flux is defined as the rate of heat flow per unit area into or out of a surface. Because of the increasing scrutiny of energy use, measurement of heat flux is becoming more important. Heat flux sensors are consequently coming into much more demand. Current heat flux sensors with manufacturers are listed in the review paper by T. E. Diller, published in 2014, titled “Heat Flux”. Diller, T. E., “Heat Flux,” Ch. 67, in Measurement, Instrumentation and Sensors Handbook, Eds. J. G. Webster and H. Eren, CRC Press, Boca Raton, Fla., 2014, pp. 67.1-15.
Because most heat flux sensors are painstakingly made by hand, the need for mass production is becoming apparent. The current invention is designed to meet this need by using a semi-automated process to produce reliable and reproducible sensors in larger quantities at much lower cost. This represents a major disruption of the commercial market and provides the opportunity to greatly expand the application of heat flux sensors for the benefit of industry and society.
As described in the ASTM standard (2684-09) for measuring heat flux at a surface, most of the methods use temperature measurements normal to the surface by placing a sensor that measures a temperature difference on the surface. ASTM E2684-09, Standard Test Method for Measuring Heat Flux Using Surface-Mounted One-Dimensional Flat Gages. Ann. Book ASTM Standards, 15.03, 2009. As with any good sensor design, the goal for good measurements must be to minimize the disruption caused by the presence of the sensor. As described in the review paper by T. E. Diller published in 2013, it is particularly important to understand the thermal disruption caused by the sensor because it cannot be readily visualized and because all heat flux sensors have a temperature change associated with the measurement. Diller, T. E., “Heat Flux Measurement,” Ch. 18, in Handbook of Measurement in Science and Engineering, Ed. M. Kutz, John Wiley & Sons, N Y, 2013, pp. 629-659. Consequently, wise selection of the sensor design and operating range is important for good heat flux measurements.
A simple heat flux sensor concept for mounting on a surface involves a layer of thermal resistance with a temperature sensor T1 on one side and a second temperature sensor T2 on the opposing side of the sensor, which has a sensor thickness δ. The one-dimensional heat flux perpendicular to the surface is found from Equation 1 for steady-state conditions:
                              q          ″                =                              k            δ                    ⁢                      (                                          T                1                            -                              T                2                                      )                                              Equation        ⁢                                  ⁢        1            
The thickness of the sensor 6 and thermal conductivity k are not known with sufficient accuracy for any particular sensor to preclude direct calibrations of each sensor. An adhesive layer may also be required between the sensor and surface to securely attach the sensor, which adds an additional thermal resistance and increases the thermal disruption. Temperature measurements on the sensor and on the surrounding undisturbed material are recommended to quantify this disruption. The ASTM standard listed gives guidance on the use of these sensors.
Although the temperature difference can be measured by any number of methods, the most commonly used are thermocouples. Thermocouples have the advantage that they generate their own voltage output corresponding to the temperature difference between two junctions. Consequently, they can be connected in series to form a thermopile that amplifies the output from a given temperature difference, which is the method described in U.S. Pat. No. 3,607,445 issued to Hines. Most any pair of conductors that are thermoelectrically different (e.g., copper-constantan) can be used for the legs of the thermopile, but the output leads should be of the same material so that additional thermocouple junctions are not created. The voltage output, E, is simply:E=NST(T1−T2)  Equation 2:
N represents the number of thermocouple junction pairs, and ST is the Seebeck coefficient or thermoelectric sensitivity of the materials, expressed in volts per degree Centigrade. The corresponding sensitivity of the heat flux sensor is:
                    S        =                              E                          q              ″                                =                                                    NS                T                            ⁢              δ                        k                                              Equation        ⁢                                  ⁢        3            
Although the sensitivity is determined in practice from a direct calibration, the last part of the equation can be used to determine the effects of different parameters for design purposes.
Examples of one-dimensional flat (or planar) sensors can be categorized based on their basic design and construction. Some gages were made by wrapping thermocouples around a resistive layer. Examples of patents incorporating these designs exist. U.S. Pat. No. 6,186,661, issued to Hevey et al., used constantan wire partially plated with copper, and U.S. Pat. No. 3,607,445, issued to Hines, used metal films that were butt-welded on each side.
Another example involves gages made by layering thermocouple and resistive layers onto one side of a substrate. The deposition process can be different; however, this design is illustrated by several patents. U.S. Pat. No. 4,779,994 issued to Diller et al. used sputtering, U.S. Pat. No. 6,278,051 issued to Peabody used metallic inks, U.S. Pat. No. 5,990,412 issued to Terrell used metallic inks, U.S. Patent Application No. US20040136434 of Langley used laminated layers, U.S. Patent Application No. US 20050105582 A1 of Thery used attached layers, and U.S. Patent Application No. US 20150308906 A1 of Durer et al. used semi-conductor materials mounted onto the substrate.
Further example Gages made with a separate temperature measurement on either side of the thermal resistive layer use RTD's (resistance temperature devices). They are not as useful for measuring heat flux as thermocouples, but have been patented by Epstein et al. in U.S. Pat. No. 4,722,609, Hayashi et al. in U.S. Pat. No. 4,577,976, and Jae-Wook Yoo in U.S. Pat. No. 8,104,952 B2.
Yet another example, thermocouples are deposited along a surface, the heat flux is measured along the surface, rather than perpendicular to it. These are generally not useful for measuring heat flux to or from a surface. Patents based on this concept include U.S. Pat. No. 6,821,015 issued to Hammer, U.S. Pat. No. 5,393,351 issued to Kinard et al., U.S. Pat. No. 9,127,988 issued to Ikeda et al., and U.S. Pat. No. 8,016,480 B2 issued to Lozinski.
A heat flux gage made by plating thermocouple materials through holes in a thermal resistance layer to connect from one side to the other have been used with copper and nickel for the thermocouple materials. The patent by U.S. Pat. No. 4,198,738 issued to Degenne discussed using metal coatings of orifices in a substratum connecting to plates on either side. The resulting gages (using copper and nickel coatings on 2.5 mm thick sheets of epoxy glass), however, were impractical because of their large size and low sensitivity. Degenne, M. and Klarsfeld, S., “A New Type of Heat Flowmeter for Application and Study of Insulation and Systems,” in Building Applications of Heat Flux Transducers, ASTM STP 885, Eds. E. Bales, M. Bomberg, and G. E. Courville, ASTM, Philadelphia, 1985, pp. 163-171.